Microwave heating of foods in a microwave oven differs significantly from conventional heating in a conventional oven. Conventional heating involves surface heating of the food by energy transfer from a hot oven atmosphere. In contrast, microwave heating involves the absorption of microwaves which may penetrate significantly below the surface of the food. In a microwave oven, the oven atmosphere will be at a relatively low temperature. Therefore, surface heating of foods in a microwave oven can be problematical.
A susceptor is a microwave responsive heating device that is used in a microwave oven for purposes such as crispening the surface of a food product or for browning. When the susceptor is exposed to microwave energy, the susceptor gets hot, and in turn heats the surface of the food product.
Conventional susceptors have a thin layer of polyester, used as a substrate, upon which is deposited a thin metal film. For example, U.S. Pat. No. 4,641,005, issued to Seiferth, discloses a conventional metallized polyester film-type susceptor which is bonded to a sheet of paper. Herein, the word "substrate" is used to refer to the material on which the metal layer is directly deposited, e.g., during vacuum evaporation, sputtering, or the like. A biaxially oriented polyester film is the substrate used in typical conventional susceptors.
In order to provide some stability to the shape of the susceptor, the metallized layer of polyester is typically bonded to a support member, such as a sheet of paper or paperboard. Usually, the thin film of metal is positioned at the adhesive interface between the layer of polyester and the sheet of paper.
Conventional metallized polyester film cannot, however, be heated by itself or with many food items in a microwave oven without undergoing severe structural changes: the polyester film, initially in a flat sheet, may soften, shrivel, shrink, and eventually may melt during microwave heating. Typical polyester melts at approximately 220.degree.-260.degree. C.
During heating, it has been observed that conventional metallized polyester susceptors will tend to break up during heating, even when the metallized polyester is adhesively bonded to a sheet of paper. Such breakup of the metallized polyester layer reduces the responsiveness of the susceptor to microwave heating. A conventional thin film susceptor becomes more transmissive and less reflective to microwave radiation during heating, as a result of breakup. A conventional thin film susceptor will typically exhibit less absorption to microwave radiation after heating. The responsiveness of the conventional susceptor to microwave radiation decreases significantly as a result of breakup.
Conventional susceptors undergo non-reversible structural and electrical changes when they are used in a microwave oven. The reduction in the microwave absorbance of the susceptor, and the consequent diminished ability of the susceptor to heat the food, is irreversible. Because breakup causes the susceptor to become more microwave transparent, it typically results in an undesirable degree of dielectric heating of the food which may, for example, lead to toughening of breadstuffs and meat.
There has been a long felt need to overcome the deleterious effects of susceptor breakup, which may adversely affect the food to be browned, crispened or otherwise heated in the presence of a microwave susceptor. There has also been a need for a susceptor which becomes substantially more microwave reflective at elevated cooking temperatures. There has been a further need for a susceptor which undergoes self-limiting microwave absorption at elevated cooking temperatures to provide a temperature controlled, thermostated crisping surface, but which remains highly reflective to microwave radiation.
Various attempts have been made in the past to provide microwave absorbing materials having a maximum temperature limit which can be attained when the material is subjected to microwave radiation. Early attempts relied upon the Curie effect, and used ferromagnetic materials for heating in response to the magnetic component of the microwave energy field.
The Curie effect may be generally described as follows. Certain microwave absorbing materials, specifically ferrites, have a Curie temperature, which theoretically provides an upper temperature limit that can be attained when the magnetic component of microwave radiation is used for heating. When the Curie temperature is reached, the ferrite material stops heating in response to the magnetic component of the microwave field, because the magnetic loss factor .mu." (the imaginary part of the complex magnetic permeability) essentially goes to zero. Prior attempts to use the Curie effect for temperature limited heating applications have generally sought to minimize the heating effects of the electric component of the microwave field. A material which exhibits the Curie effect may, however, continue to heat above the Curie temperature if the electric loss factor .epsilon." is significant and the local electric field is appreciable.
An early example of an attempt to use the Curie effect is shown by U.S. Pat. No. 2,830,162, issued to Copson et al. However, Copson et al. teach that the material being heated to its Curie temperature becomes more transmissive--"any further R. F. energy thereafter received being transmitted as R. F. energy without significant loss." See column 1, lines 57-60 (emphasis added). Thus, Copson et al. fail to disclose a microwave susceptor which becomes substantially more reflective at elevated cooking temperatures.
An effort to achieve a self-limiting temperature is shown in U.S. Pat. No. 4,266,108, issued to Anderson et al. The Anderson et al. reference discloses a microwave absorption material which uses the magnetic component of the microwave energy for heating instead of the electrical component of the microwave energy. The Anderson et al. reference describes as a "problem": how to provide a device which would utilize the magnetic field component of the microwave energy as a source of energy for heating, while substantially excluding the electrical field component from providing energy for heating, in order to prevent thermal runaway. See column 4, lines 29-34.
The solution proposed by Anderson et al. involved placing a metallic electrically conductive surface, such as a sheet of metal, immediately next to the microwave absorbing material. At such a conducting surface, the magnetic component of the microwave field is maximum while the electric field component is at a node, or is minimal. As taught by Anderson et al., "little or no energy is available to the absorbing material from the electric field component." See column 4, lines 40-68. Anderson et al. also taught the use of materials which did not change electrical resistivity with temperature. For example, see the table at column 5, beginning at line 23. The value for .epsilon." was 0.76 at room temperature, and was 0.76 above 255.degree. C. .epsilon." can be converted to a value of conductivity, or alternatively to a value of resistivity. From the value given for .epsilon." in the table disclosed by Anderson et al., it can be seen that the resistivity did not change with temperature. The total susceptor structure disclosed by Anderson et al. had a transmittance of zero, because the metallic reflective surface did not permit microwave radiation to be transmitted through the composite structure.
Efforts to use the Curie effect and heating based upon the magnetic component of the microwave field have been limited by the fact that the magnetic loss factor .mu." of practical materials is of a relatively small magnitude. A much larger magnitude of the electric loss factor .epsilon." is available in practical materials, and in accordance with the present invention can be used to provide much more effective temperature dependent heating control than prior Curie effect approaches. In addition, because the magnetic loss factor .mu." is small, practical devices require thick layers of material to achieve significant microwave absorption and these magnetic devices, therefore, tend to be expensive.
Similarly, U.S. Pat. No. 4,190,757, issued to Turpin et al., shows the use of Curie temperature with ferromagnetic materials as the microwave absorbing material.
Turpin et al. state that any suitable lossy substance that will heat in bulk to more than 212.degree. F. may be used as the active heating ingredient of the microwave energy absorbent layer 46. They then provide a list of suggested substances, which includes: dielectric materials such as asbestos, some fire brick, carbon and graphite; and period eight oxides and other oxides such as chromium oxide, cobalt oxide, manganese oxide, samarium oxide, nickel oxide, etc.; and ferromagnetic materials such as powdered iron, some iron oxides, and ferrites including barium ferrite, zinc ferrite, magnesium ferrite, copper ferrite, or any of the other commonly used ferrites and other suitable ferromagnetic materials and alloys such as alloys of manganese, tin and copper or manganese, aluminum and copper and alloys of iron and sulfur, such as pyrrhotite with hexagonal crystals, etc., silicon carbide, iron carbide, strontium ferrite and the like; and, what are loosely referred to as "semiconductors", examples of which are given as zinc oxide, germanium oxide, and barium titanate.
Turpin et al. fail to teach or suggest a susceptor which is transmissive, and which becomes substantially more microwave reflective at elevated temperatures. Turpin et al. use a metal sheet as a support layer 44 for the food product in the claimed preferred embodiment. In such an example, the composite structure would have virtually no transmission of microwave energy. The layer 44 is also suggested as alternatively comprising a nonmetal mineral or a thin glaze of ceramic fused to the upper surface of the heat absorbing layer 46. In this example, the composite structure would not become more reflective as the result of microwave heating.
U.S. Pat. No. 4,808,780, issued to Seaborne, discloses compositions for a ceramic utensil to be used in microwave heating of food items. The compositions include certain metal salts as time and temperature profile moderators in addition to microwave absorbing material and a binder. Certain metal salts are used to dampen or lower the final temperatures reached upon microwave heating of the ceramic composition. Other metal salts are used to increase or accelerate the final temperature reached upon microwave heating. The accelerators are divided into two groups, some of the accelerators being identified as super accelerators which exhibit a markedly greater acceleration effect. Seaborne then goes on to give a list of materials which he states are useful in this particular limited application.
Seaborne states that exemplary useful dampeners are selected from the group consisting of MgO, CaO, B.sub.2 O.sub.3, Group IA alkali metal (Li, Na, K, Cs, etc.) compounds of chlorates (LiClO.sub.3, etc.), metaborates (LiBO.sub.2, etc.), bromides (LiBr, etc.), benzoates (LiCO.sub.2 C.sub.6 H.sub.5, etc.), dichromates (Li.sub.2 Cr.sub.2 O.sub.7, etc.), all calcium salts, SbCl.sub.3, NH.sub.4 Cl, CuCl.sub.2, CuSo.sub.4, MgCl.sub.2, ZnSO.sub.4, Sn(II) chloride, vanadyl sulfate, chromium chloride, cesium chloride, cobalt chloride, nickel ammonium chloride, TiO.sub.2 (rutile and anatase), and mixtures thereof. Seaborne says that exemplary useful accelerators are selected from the group consisting of Group 1A alkali metals (Li, Na, K, Cs, etc.) compounds of chlorides (LiCl, etc.), nitrites (LiNO.sub.2, etc.), nitrates (LiNO.sub.2, etc.), iodides (LiI, etc.), bromates (LiBrO.sub.3, etc.), fluorides (LiF, etc.), carbonates (LiI, etc.), phosphates (Li.sub.3 PO.sub.4, etc.), sulfites (Li.sub. SO.sub.3, etc.), sulfides (LiS, etc.), hypophosphites (LiH.sub.2 PO.sub.2, etc.), BaCl.sub.2, FeCl.sub.3, sodium borate, magnesium sulfate, SrCl.sub.2, NH.sub.4 OH, Sn(IV) chloride, silver nitrate, TiO, Ti.sub.2 O.sub.3, silver citratre and mixtures thereof. Seaborne further states that "super accelerators" are selected from the group consisting of B.sub.4 C, ReO.sub.3 CuCl, ferrous ammonium sulfate, AgNO.sub.3, Group 1A alkali metals (Li, Na, K, Cs, etc.), compounds of hydroxides (LiOH, etc.), hypochlorites (LiOCl, etc.), hypophosphates (Li.sub.2 H.sub.2 P.sub.2 O.sub.6, Na.sub.4 P.sub.2 O.sub.6, etc.), bicarbonates (LiHCO.sub.3, etc.), acetates (LiC.sub.2 H.sub.3 O.sub.2, etc.), oxalates (Li.sub.2 C.sub.2 O.sub.4, etc.), citrates (Li.sub.3 C.sub. 6 H.sub.5 O.sub.7, etc.), chromates (Li.sub.2 CrO.sub.4,e tc.), and sulfates (Li.sub.2 SO.sub.4,e tc.), and mixtures thereof. Other exemplary useful accelerators listed by Seaborne are certain highly ionic metal salts of sodium, magnesium, silver, barium, potassium, copper, and titanium, including, for example, NaCl, NaSO.sub.4, AgNO.sub.3, NaHCO.sub.3, KHCO.sub.3, MgSO.sub.4, sodium citrate, potassium acetate, BaCl.sub.2, KI, KBrO.sub.3, and CuCl. The most preferred accelerator identified by Seaborne is common salt due to its low cost and availability. See column 7, line 55 to column 8, line 23.
Seaborne failed to discover that certain materials can be used to make a susceptor which becomes substantially more microwave reflective at elevated cooking temperatures, and which have a microwave interactive heating layer whose conductivity increases with increasing temperature.
In the description contained herein, the term "semiconductor" is used to refer to material which is commonly known as semiconductor material, such as silicon and germanium. Semiconductors are a class of materials exhibiting electrical conductivities intermediate between metals and insulators. These intermediate conductivity materials are characterized by the great sensitivity of their electrical conductivities to sample purity, crystal perfection, and external parameters such as temperature, pressure, and frequency of the applied electric field. For example, the addition of less than 0.01% of a particular type of impurity can increase the electrical conductivity of a typical semiconductor like silicon and germanium by six or seven orders of magnitude. In contrast, the addition of impurities to typical metals and semimetals tends to decrease the electrical conductivity, but this decrease is usually small. Furthermore, the conductivity of semiconductors characteristically increases, sometimes by many orders of magnitude, as the temperature is increased. On the other hand, the conductivity of metals and semimetals characteristically decreases when the temperature is increased, and the relative magnitude of this decrease is much smaller than are the characteristic changes for semiconductors. See the Encyclopedia of Physics, (2d ed. 1974), edited by Robert M. Besancon and published by Van Nostrand Reinhold Company, pages 835-42 of which are incorporated herein by reference.
In some prior patent descriptions, the term "semiconductive" has been given a different meaning. In some published patent descriptions, thin metal films have been referred to as "semiconductive" in an attempt to describe the fact that the thin film had a measurable surface resistance and would heat when exposed to microwave radiation. An example of this is shown in U.S. Pat. No. 4,267,420, issued to Brastad, where it is said "for the lack of a completely definitive generic word in the broader claims, the term `semiconducting` will be used." See column 5, lines 28-30. See also U.S. Pat. No. 4,735,513, issued to Watkins et al., at column 5, lines 36-45; U.S. Pat. No. 4,825,025, issued to Seiferth, at column 1, lines 37-37; U.S. Pat. No. 4,230,924, issued to Brastad et al., at column 6, lines 24-28; U.S. Pat. No. 4,777,053, issued to Tobelmann. Thin films of metals such as aluminum, chromium, silver, gold, etc., are not intended to be included in the meaning of the term "semiconductor" as used herein. In the description below of the present invention, the term "semiconductor" is used in accordance with its traditionally accepted meaning to refer to semiconductors like germanium and silicon. The present invention is particularly concerned with semiconductors whose conductivity increases with temperature.
U.S. Pat. No. 4,283,427, to Winters et al., discloses a lossy chemical susceptor which, upon continued exposure to microwave radiation, eventually becomes substantially microwave transparent. Other patents uncovered during a prior art search which provide a general background of the prior art are U.S. Pat. Nos. 4,691,186, to Shin et al., 4,518,651, to Wolfe, Jr., 4,236,055, to Kaminaka, and 3,853,612, to Spanoudis.
It is clear from the above description that conventional susceptors have exhibited problems and drawbacks, and have not been fully satisfactory for all applications and purposes. The need for a susceptor operative to brown and crispen the surface of food, but which does not exhibit the deleterious effects of breakup, and which becomes substantially more microwave reflective and less absorptive at elevated cooking temperatures, is apparent.